In the context of automotive finishing, a wide variety of different substrates, and more particularly metallic substrates, are coated generally with multiple coats, in order to meet the various technological performance requirements that are imposed on the coated substrates, such as the corrosion resistance, for example. The coatings involved are generally multicoat systems comprising—starting from the metallic substrate—a conversion coating (phosphate coating, for example), and also an electrocoat, a primer-surfacer coat, a basecoat, and a clearcoat. A less complex and hence more economical coat system would be an advantage in this context. In that case, of course, it will still be necessary for the technological performance properties of the overall coating to meet the exacting requirements of the automobile industry.
Attempts are being made, for example, to replace the corrosion-protection electrocoat and the primer-surfacer coat by a single coating that unites the properties of the two aforementioned coats. A particular problem in this context is to maintain appropriate corrosion protection and to attain an acceptable adhesion and hence water and moisture resistance on the part of the overall coating. If attempts are made not only to unite electrocoat and primer-surfacer coat but also to do without the conversion coat, in order to make the coating and the coating process more economical and more quick to implement, the problems in terms of corrosion resistance are intensified still further. For example, the adhesion of substitute coats (primer coats) applied direct to the metallic substrate (direct-to-metal) is often not sufficient to ensure maintenance of the coat even under long-term weathering effects, more particularly moisture effects.
The problems addressed are of relevance not only in the context of the original (OEM) finishing of substrates—such as, for example, of substrates which are painted as part of automotive OEM finishing. These problems must also be paid particular attention in the refinish segment, more particularly automotive refinishing. On the one hand, in this segment particularly, a reduction in the number of individual coats to be applied is an advantage, allowing the refinish operation to be conducted more rapidly and economically. On the other hand, especially with refinishing, where regions with locally damaged original finish are surrounded by intact zones of the original finish, the adhesion achieved is often inadequate. This is especially the case if any residues of the original finish, along with the corrosion products that have usually already developed, must be removed from the damaged site by cleaning and abrading, and the substrate surface is exposed as a result. The adhesion problems arise not least from the different interfaces. First, here, there is the interface between the refinish and the exposed substrate. Moreover, the newly applied finish must also adhere to the corresponding interfaces and boundary edges in the region between the damaged, cleaned, and abraded areas and also the regions with intact original finish that surround these areas. At all of these interfaces, a single coating composition must ensure adequate adhesion. The provision of a direct-to-metal primer composition which can be used in refinishing and which affords appropriate corrosion protection is therefore an even greater challenge.
Coating compositions with which attempts are made to counter the stated problems, especially in respect of deficient adhesion and deficient corrosion protection in connection with the direct-to-metal primer coats described, are described in patent application US2006/0047085, for example. The coating compositions described therein lead to acceptable corrosion resistance and weathering resistance, but contain relatively large quantities of organic solvents.
Nowadays, however, specific upper limits on emissions of volatile organic constituents, especially solvents, are laid down by law for the operation of industrial painting plants. Moreover, compliance with the statutory stipulations must be adequately demonstrated (cf., for example, the 31st German Federal Airborne Pollutants Ordinance, and also the corresponding VOC Guidelines and VOC Ordinances of the EU). Particularly as part of what is called simplified reduction plan, developed specifically for painting plants in the lower size segment (automotive refinish, for example), it can be assumed that the statutory approval for the operation of such plants would in future be achievable through the use of coating compositions having a VOC of less than 250 g/l. For coating compositions in the form in which they have been formulated ready for use, the VOC is defined as follows: VOC (g/l)=(mass of volatiles (g)−mass of water (g))/(volume of coating material (l)−volume of water (l)). Volatiles here are those compounds which at processing temperature, more particularly 20° C., exceed a vapor pressure of 10 pascals. They include, in particular, the commonly known organic solvents, examples being the organic solvents specified later on below, and also water. Coating compositions which in the ready-to-process state have a low VOC level of this kind in general have only a small amount of organic solvents and, moreover, a high fraction of water in comparison to the organic solvents.